This invention relates generally to screens and more particularly concerns screens used in vibrating screening machines to remove undesirable particles from a liquid such as oil well drilling fluids.
The factors of primary importance in screens are particle separation, fluid throughput or conductance, and screen life. Finer particle separation results in a higher percentage of impurities being removed from the screened fluid. Higher conductances are desirable because more fluid can be processed per square foot of screen area, thereby improving treatment efficiency and reducing costs. As an example, doubling the conductance doubles the liquid throughput. Longer screen life means that the screens have to be replaced less frequently, thereby saving time and money.
Since the mid-1970s, the trend in the vibrating screen industry has been to decrease the diameter of the wires comprising the screens in order to increase conductance. The result has been finer separation and higher conductance but shorter screen life. Undesirable particles can still become trapped among the wires of the screen, reducing its conductance. In addition, screens made of thinner wires have less resistance because thinner wires are more prone to deformation from parallel. This non-parallel deformation creates openings in the screen that have widths that are either smaller or greater than expected and allows undesirable particles to pass through the screen.
Therefore, it is an object of this invention to provide a triple-layer screen where at least one of the layers uses coarser wire than conventional screens. It is another object of this invention to maximize conductance through the screen while maintaining screen-life that is at least comparable to conventional screens. It is another object of this invention to reduce the number of undesirable particles that become entrapped in the upper layer of the screen, further enhancing the screen's conductance. It is another object of this invention to lengthen the effective life of the screen.